The recirculation of flue gases is a technology which can basically be used for the most diverse purposes in gas turbines, such as for the control of emissions, for the reduction of the flue gas volume, for carbon dioxide separation, etc. During the recirculation of flue gases in a gas turbine, an essential portion of the flue gas is tapped from the overall flue gas flow and, after cooling and scrubbing, is normally returned to the inlet mass flow of the turbine or to the compressor of the turbine, wherein the returned flue gas flow is mixed with fresh air from the environment, and this mixture is then fed to the compressor.
As a result of flue gas recirculation, the carbon dioxide partial pressure in the flue gases can advantageously be increased in order to reduce the power losses and efficiency losses of power plants with carbon dioxide separation. Furthermore, flue gas recirculation has been proposed with the aim of reducing the oxygen content in the intake gases of gas turbines in order to reduce the NOx emissions as a result.
For flue gas recirculation, U.S. Pat. No. 7,536,252 B1, for example, describes a method for controlling an flue gas recirculation flow of a turbomachine which is fed back via an flue gas recirculation system to the intake of the turbomachine. With this method, a nominal flue gas recirculation portion, which contains the portion of the flue gas flow in the inlet flow of the turbomachine, is specified, and the actual value is adjusted to the nominal value.
A power plant with flue gas recirculation and also a method for operating such a power plant is known from EP2248999, in which, in dependence upon load, the recirculation rate and the temperature to which the recirculated flue gases are recooled are controlled.
It is known to achieve a recirculation rate which is as high as possible. The recirculation rate is generally restricted by the requirements of the combustion chamber, since otherwise the oxygen content of the combustible gases becomes too low and a complete CO-free and UHC (unburnt hydrocarbon)-free combustion cannot be ensured.
It is known that in order to ensure good combustion in the case of low oxygen content in the combustible gases, the fresh air is mixed with the recirculated flue gases as thoroughly as possible in order to provide a homogenous gas mixture upon entry into the combustion chamber. A mixer which is suitable for low pressure-loss mixing of fresh air with recirculated flue gases is known from WO2010/142573 A2, for example.
Despite the feed of a homogeneous gas mixture, the residual oxygen content of the flue gases of such a plant is still relatively high, since in known gas turbines oxygenous gases, by means of the cooling air system, are routed past the combustion chamber and in the turbine or in the combustion chamber are supplied as cooling air without the oxygen contained therein being able to be used for combustion.